a) Field of the Invention
The present invention relates to vehicular brake controlling apparatus in which a braking liquid pressure for rear road wheels is executed to prevent a lock of the rear road wheel(s) earlier than front road wheel(s) during a braking operation of the vehicle.
b) Description of the Related Art
Since a wheel load of each front road wheel(s) is Increased whereas that of each rear road wheel(s) is decreased during a vehicular braking, there is a strong tendency of the rear road wheel(s) as compared with the case of the front road wheel(s). Hence, if the rear road wheel(s) should be locked at an earlier time than the front road wheel(s), a vehicular behavior would become unstable.
In order to equalize the lock tendencies between the rear and front road wheels, it is known that the braking force is acted upon the front and rear road wheels in accordance with an ideal braking force distribution. Hence, in general, a proportioning valve is interposed in a braking conduit to raise the braking liquid pressure for the front and rear road wheels on the basis of the ideal braking force distribution.
Recently, various types of vehicular anti-skid (or anti-lock) brake control systems (so-called, ABS) to prevent the vehicular road wheel locks from occurring have been mounted in many automotive vehicles.
A Japanese Patent Application First Publication No. Heisei 5-278585 exemplifies one of the anti-lock brake control systems using a brake unit capable of executing the ABS control to achieve the same function as the proportioning valve. The Japanese Patent Application First Publication identified above teaches a braking force distribution control such that when the revolution velocity of the rear road wheel(s) is slower than that of the front road wheel(s), a rear-road-wheel brake actuator is activated to suppress a boosting of a braking liquid pressure for the rear road wheel(s) such as a pressure decrease or hold of a pressure and, when the revolution velocity of the front road wheel(s) is faster than that of the rear road wheel(s), the rear-road-wheel brake actuator is activated to recover the brake liquid pressure.
On the other hand, the brake controlling apparatus in which the braking force distribution control using the brake unit is executed has been proposed in which a predetermined back-up control is executed when a failure in the brake controlling apparatus occurs. For example, the execution of the braking force distribution control is inhibited during a detection of the failure (refer to a Japanese Patent Application First Publication No. Heisei 6-211116 published on Aug. 2, 1994).
Furthermore, a German Patent Application First Publication No. DE-A1 44 14 980 published on Nov. 2, 1995 exemplifies another previously proposed vehicular brake controlling apparatus in which the braking force distribution control is continued on the basis of any signals derived from vehicular road wheel revolution velocity sensors which function normally when the failure of any one of the road wheel revolution velocity sensors occurs and a left-and-right road wheel synchronous control is advanced when executing the braking force distribution control for the rear road wheels in the case where either of the rear road wheel revolution velocity sensors has failed.